"However, notice that the value following TIME ZONE is only allowed to
be an interval by the spec (and this is still true in SQL:2021, the latest version I have handy). Such an interval is interpreted per ISO (positive = east of Greenwich)."
Erm, what do you mean by an interval? If you mean a number, then it’s broken, because "UTC+10" and "+10" do the same thing. But you seem to be saying there is indeed some syntax that is interpreted by ISO logic?
There is a named data type called “interval”. He’s referring to that. Neither of those text values is an interval. ‘4 hours 30 minutes’::interval is a relevant example.
